Some have already answered this, but I'd like to add a bit and summarize.

**1)** With optical **you first need to actually fire a flash**. Sometimes you can fire the built-in flash. But you will contaminate the scene with that first flash. You do not want that when you really want a mood.

Radio trigger = Problem solved.

**2)** Optical slaves do not fire in some **bright situations**, not only in daylight, but also indoors where some spotlight is over one of your slaves.

Radio trigger = Problem solved.

**3)** Optical. You need a **line of sight**. This limits you on the setup.

Radio trigger = Problem solved.

**4)** Distances. With optical, the slave won't detect at some **range**. Let's say 5 m.

Radio trigger = Problem solved. Some triggers work with ranges at least 10 times larger.

You can have a flash on the other side of a room and trigger it.

**5)** **Flash firing contamination**. You are at a wedding with your optical flashes...and everyone is firing their own flashes... you are in a trouble.

Radio trigger = Problem solved.

**6)** In some models you can **remotely adjust the intensity** of the flash from the trigger itself.

You can set up 3 groups for example, and change group A to 1/2 power, group B to 1/4, and group C to 1/8 or whatever combination you want. No need to run to your flashes.

**7)** And in some models you also have TTL. **TTL on an offshoe flash**!

**Flexibility** That is the difference.

---

Nowadays there are some cheap but sturdy radio triggers and receivers, and some flashes with radio receivers built in. Price is not a real issue anymore.